High ionic conductivity of Mg–Al layered double hydroxides at intermediate temperature (100–200 °C) under saturated humidity condition (100% RH)

Ionic conduction properties of layered double hydroxides (LDHs), [Mg(1 − x)Al(x)(OH)2](CO3)x/2·nH2O (═CO3-type) and [Mg(1 − x)Al(x)(OH)2](OH)x·nH2O (═OH-type), have been investigated in the temperature range of 25–200 °C under saturated water vapor pressure (100% RH). Conductivity was measured by the complex impedance method using pressed pellet samples. The conductivity of an OH-type sample (x = 0.25) is as high as 4 × 10− 3 S cm− 1 already at room temperature. On heating, it increases as slowly (the activation energy Ea ∼ 55 meV) as those of liquid electrolyte solutions, suggesting that ions are transported via exchange of protons between OH− and H2O species in the interlayer space of the LDH (a kind of the Grotthus mechanism). The conductivity at 200 °C reaches 3 × 10− 2 S cm− 1. Although the conductivity of the CO3-type counterpart is considerably low (∼ 10− 6 S cm− 1) at room temperature, it increases with increasing temperature with showing a jump by about four orders of magnitude around 80 °C, after which the compound is turned into an almost the same liquid-like conductor as the OH-type sample; the conductivity jump is probably due to the interlayer reaction CO32− + H2O = 2OH− + CO2 (g). Thus those Mg–Al LDH materials show the potential application as solid electrolyte materials for intermediate temperature fuel cells, not only because they have high conductivity but also they should not suffer from the degradation by CO2 which is a serious problem for conventional anion exchange membranes.